Sitemaps settings page enhancements

[tburry]

Let's add an enhancement for the sitemaps.

Change the wording for the settings description to:

A site map is an xml file that search engines can use to help index your site. Learn more about sitemaps at sitemaps.org.

• Your main site map is located here: http://dev.vanilla.localhost/sitemapindex.xml
• Your robots.txt file contains the location of this site map so that search engines that are aware of your site will know where to look.

Add a toggle to the settings page to enable the feature flag

Label: Discussion Based Sitemaps (BETA)
Description: Use the sitemaps that point directly to discussions instead of categories.